1. Field of the Invention
This invention relates to a current limiting arrangement for general circuit protection including electrical distribution and motor control applications. In particular, the invention relates to a current limiting arrangement that is capable of limiting the current in a circuit when a high current condition occurs.
2. Description of Related Art
There are numerous devices that are capable of limiting the current in a circuit when a high current condition occurs. One known limiting device includes a filled polymer material which exhibits what is commonly referred to as a PTCR (positive-temperature coefficient of resistance) or PTC effect. U.S. Pat. Nos. 5,382,938, 5,313,184, and European Published Patent Application No. 0,640,995 A1 each describe an electrical device relying on PTC behavior. The unique attribute of the PTCR or PTC effect is that at a certain switch temperature the PTCR material undergoes a transformation from a basically conductive material to a basically resistive material. In some of these prior current limiting devices, the PTCR material (typically polyethylene loaded with carbon black) is placed between pressure contact electrodes.
U.S. patent application Ser. No. 08/514,076, filed Aug. 11, 1995, now U.S. Pat. No. 5,614,881 issued Mar. 25, 1997, the entire contents of which are herein incorporated by reference, discloses a current limiting device. This current limiting device relies on a composite material and inhomogeneous distributions of resistance structure.
Current limiting devices are used in many applications to protect sensitive components in an electrical circuit from high currents. Applications range from low voltage and low current electrical circuits to high voltage and high current electrical distribution systems. An important requirement for many applications is a fast current limiting response to minimize the peak fault high current that develops.
In operation, current limiting devices are placed in a circuit to be protected. Under normal circuit conditions, the current limiting device is in a highly conducting state. When a high current condition occurs, the PTCR material heats up through resistive heating until the temperature is above the xe2x80x9cswitch temperaturexe2x80x9d. At this point, the PTCR material resistance changes to a high resistance state and the high current condition current is limited. When the high current condition is cleared, the current limiting device cools down over a time period, which can be long, to below the switch temperature and returns to the highly conducting state. In the highly conducting state, the current limiting device is again capable of switching to the high resistance state in response to future high current condition events.
Known current limiting devices include conducting composite material comprising at least one of a low pyrolysis or vaporization temperature polymeric binder and an electrically conducting filler combined with inhomogeneous distributions of resistance structure. The switching action of these current limiting devices occurs when joule heating of the electrically conducting filler in the relatively higher resistance part of the composite material causes sufficient heating to cause pyrolysis or vaporization of the binder, that occurs at a predetermined point in time after the switching occurs.
A conductive composite material possesses electrical and physical characteristics that define the operational limits or bounds for a current limiting device. For an optimum operation of a current limiting device in an intended use and application, a conductive composite material should possess many desirable criteria and properties in order to enable a wide range of uses. The desirable criteria and properties include, but are not limited to, a low bulk and contact resistance so as to have an adequate low power dissipation when the current limiting device is in an unswitched state; a relatively short switching time and a relatively high switched resistance during a high current condition event to adequately limit the high current condition current; and a relatively high energy absorbing capacity to absorb high current condition energy during a high current condition event.
It is relatively difficult to obtain a single conductive composite material that satisfies all desirable criteria and properties for particular applications of a current limiting device, so the current limiting device has satisfactory operational bounds. A single composite material may possess one or more of the desirable criteria and properties for an intended use of the current limiting device, however it is not likely that a single conductive composite material possesses all desired criteria and properties for a particular application and intended use of a current limiting device, so the current limiting device has satisfactory operational bounds.
For example, but not limited to, a single conductive composite material will possess a low bulk and contact resistance to have an adequate low power dissipation when the current limiting device is in an unswitched state. However, it is unlikely that the single conductive composite material will possess a relatively short switching time or a relatively high switched resistance during a high current condition event to adequately limit the high current condition current. Another conductive composite material will possess a relatively high energy absorbing capacity to absorb a high current condition energy during a high current condition event, but fail to provide a low bulk and contact resistance so as to have an adequate low power dissipation when the current limiting device is in an unswitched state. Thus, the operational bounds of a current limiting device will be limited by the conductive composite material in the current limiting device, and a current limiting device that needs all of these criteria for successful operations will not be obtained.
The lack of a single conductive composite material with all desirable criteria and properties for an intended use and application of a current limiting device, of course, makes it difficult to obtain a current limiting device with optimum operational bounds. Since a single conductive composite material will not normally possess all desirable criteria and properties, known current limiting devices will have limited uses and applications, dependent on the conductive composite material.
Accordingly, it is desirable to provide a current limiting arrangement that overcomes the above, and other, disadvantages of known current limiting devices.
Accordingly, it is desirable to provide a current limiting arrangement that utilizes at least two electrically conductive composite materials, with a resistance distribution structure in a current limiting device. The multiple and distinct electrically conductive composite materials are chosen considering the intended use and application of the current limiting device, thus allowing optimization of various desirable properties and criteria for a successful current limiting operation. The operational parameter bounds for the intended use and application on each composite material in a current limiting arrangement are reduced by combining the individual desirable properties of the conductive composite materials. Since no single conductive composite material is needed to exhibit all desirable properties and criteria in a current limiting device for the intended use and application, the combination of conductive composite materials lends to a desirable current limiting arrangement.